This invention relates to a conditioning system for preparing and introducing sulfuric acid (H.sub.2 SO.sub.4) into a particle laden flue gas stream so as to substantially reduce the resistivity of the fine fly ash particulates to in turn enhance their removal from the gas stream by electrical precipitator means. The invention is an improvement upon the invention disclosed and claimed in U.S. Pat. No. 4,070,424 assigned to a common assignee.
It has been found, and is quite well known, that fly ash containing streams from the burning of coal, or from the burning of any fossil fuels, will have a certain amount of electrical resistance or "resistivity" developed in the particles such that an inefficient electrical precipitation thereby results. It is also well known that flue gas streams will have varying quantities of sulfur trioxide (SO.sub.3) present naturally and that when a sufficient quantity of SO.sub.3 or H.sub.2 SO.sub.4 is present in the gas stream or with the fly ash the resistivity of the particles to giving up their electrostatic charges will be low enough that good precipitation results can be obtained.
With ever increasing state and federal pressure on industrial and utility companies to improve their emissions from coal fired boilers and comply with emission standards, many have switched to the use of low sulfur coal to reduce the amount of SO.sub.2 present in the flue gases. Unfortunately, while the flue gas from high sulfur coal contains sufficient SO.sub.3 to provide the proper resistivity, low sulfur coal lacks sufficient SO.sub.3 in the flue gas to provide the proper resistivity to the resulting fly ash to permit its effective precipitation. Thus, these users are more than ever seeking immediate and low cost solutions to poor fly ash collection efficiencies of their existing electrostatic precipitators. Their possible options are to expand or rebuild their existing equipment to handle the type of flue being fired or by going to gas conditioning of the boiler combustion gases. Gas conditioning is used to bring the exhaust fly ash within a more desirable resistivity range for precipitator collection. Gas conditioning is economically more attractive to industry due to its relatively low purchase price when compared to the purchase price of an enlarged or new precipitator. Availability is a second advantage to this approach in that the system can be installed fairly quickly and with minimal load disturbance.
Various methods of gas conditioning are presently available. The more effective conditioning agents are H.sub.2 SO.sub.4 and NH.sub.3. A system which is presently marketed and which is described in U.S. Pat. No. 3,704,569 uses vaporized H.sub.2 SO.sub.4 as its conditioning agent. With this system, large volumes of dry air are heated to a temperature of approximately 260.degree. C. to be above the vaporizing temperature of about 235.degree. C. and then mixed with the acid in a glass lined vaporizing chamber. The hot vaporized acid is then conveyed to injection lances by means of glass lined pipe and uniformly dispersed in the flue gas. Although such a system provides excellent conditioning of the flue gases, it is quite expensive to produce due to the fact that the acid is transported in a hot vaporized state and is extremely corrosive, with the result that expensive, corrosion-resistant materials are required to be used. Furthermore, the system is expensive to operate since an excessive amount of energy must be used to heat the air to a point where it can vaporize the acid.
A second method of gas conditioning is that of utilizing SO.sub.3 directly. This system functions much the same as the aforementioned vaporizer except that heat is applied to the liquid SO.sub.3 in an evaporator chamber resulting in the SO.sub.3 vapor. The SO.sub.3 is unstable and must be heated in storage tanks, pumps and flow lines. Since any leakage of liquid becomes gaseous, the system is quite hazardous.
A third method is disclosed in U.S. Pat. No. 1,441,713 where acid is proposed to be introduced in a gas stream in the form of very fine particles and specifically, in the form of a fume which is formed by boiling fuming sulfuric acid. Although the patentee broadly contemplates that the acid be introduced by some suitable form of atomizing device, no apparatus is disclosed other than the boiling pan and burners. In view of the extremely corrosive and dangerous nature of fuming sulfuric acid, it is doubtful that the aforesaid method would have ever been used. Certainly if it was it would have been expensive to provide corrosion-resistant materials and to provide the necessary heat for boiling the acid.
A fourth and more complex method of gas conditioning is that of burning liquid sulfur. The SO.sub.2 generated by the sulfur burner is passed through a catalyst that converts the SO.sub.2 to SO.sub.3. The final objective of all four methods is to disperse H.sub.2 SO.sub.4 in the precipitator flue gases and condition same to a more desirable resistivity for precipitator collection. The dispersion must be very fine since an electrical precipitator is an effective collector of sulfuric acid mist. As noted above, conditioning usually involves the injection of H.sub.2 SO.sub.4 or SO.sub.3 in the flue gas stream in vaporized form, and the injection of acid in liquid form apparently has not been done commercially, probably for the reason that one would expect that liquid injection would not condition beyond the first field of a precipitator since the acid particles would be collected, leaving the remaining fields current suppressed due to the presence of unconditioned fly ash accumulated on the electrodes. Furthermore, until the rather recent development of sonic nozzles, the available mechanically atomized spray nozzles were not able to produce a fine enough spray to be considered as a substitute for vapor injection. Mechanical nozzles typically are poor in their ability to be turned down to low flows. Also, the required high liquid pressures and small orifices used would increase the likelihood of erosion and plugging problems.
The invention disclosed in said U.S. Pat. No. 4,070,424 was developed to avoid the disadvantages of the aforementioned systems and provides for injecting liquid acid directly into the flue gas stream. However, extensive testing has indicated that under gas flow conditions the atomized plume of sulfuric acid can collapse and cause agglomeration of the mist into droplets which can wet the internal duct structure and cause undesirable ash buildup.